It* 


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LIBRARY 


UNIVERSITY  OF  CALIFORNIA. 


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An  Investigation  of  Zinc  Amalgams  and 
Concentration  Cells 


A  THESIS 


SUBMITTED  TO  THE  FACULTY  OF  PRINCETON  UNIVERSITY  IN 

PARTIAL  FULFILMENT  OF  THE  REQUIREMENTS  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY. 


BY 


J.  L.  CRENSHAW 


EASTON,  PA.: 

ESCHENBACH  PRINTING  COMPANY. 
1911. 


An  Investigation  of  Zinc  Amalgams  and 
Concentration  Cells 


A  THESIS 


SUBMITTED  TO  THE  FACULTY  OF  PRINCETON  UNIVERSITY  IN 

PARTIAL  FULFILMENT  OF  THE  REQUIREMENTS  FOR 

THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY. 


BY 

J.  L.  CRENSHAW 


EASTON,  PA.: 
ESCHENBACH  PRINTING  COMPANY. 

I9II. 


:.  ::/-:'.::"'  •: : 


An  Investigation  of  Zinc  Amalgams  and 
Concentration  Cells 

Introduction 

Although  the  first  investigation  of  zinc  amalgams  was 
undertaken  more  than  fifty  years  ago1  and  a  vast  amount  of 
work  has  been  done  since  that  time  in  this  very  important 
field,2  nevertheless  at  the  time  the  present  research  was  under- 
taken very  few  accurate  and  no  complete  data  on  the  subject 
were  obtainable. 

In  1877  Helmholtz3  in  his  epoch-making  paper  developed 
the  theory  of  concentration  cells  from  a  thermodynamic 
point  of  view  and  twelve  years  later  Nernst,4  from  a  purely 
physical-chemical  standpoint,  worked  out  his  equation  for  the 
electromotive  force  of  concentration  cells.  G.  Meyer5  and 
von  Turin8  were  the  first  to  confirm  the  theoretical  formula 
by  the  results  of  experiment  by  working  with  amalgam  con- 
centration cells.  The  special  interest  taken  in  a  study  of 
these  cells  is  largely  due  to  the  fact  that  they  are  the  sim- 
plest kind  of  primary  cells  and  therefore  offer  the  most  prom- 
ising field  for  rigidly  verifying  our  theories  or  of  explaining 
any  observed  discrepancy  between  the  theoretical  and  experi- 
mental values  obtained. 

Previous  workers  have  restricted  themselves  to  a  rela- 
tively small  range  of  concentration  on  account  of  the  experi- 
mental difficulties  involved  in  preparing  and  handling  amal- 


1  Guigain:  Compt.  Rend.,  42,  430;  Becquerel:  Ann.  Chim.  Phys.,  48,  266 
(1856). 

2Hockinand  Taylor:  Jour.  Soc.  Tel.,  282  (1872);  Lindeck:  Wied.  Ann., 
35,  311  (1888) ;  von  Turin:  Zeit.  Phys.  Chem.,  5,  340  (1890) ;  Richards  and  Lewis: 
Zeit.  Phys.  Chem.,  28,  7;  Cady:  Jour.  Phys.  Chem.,  2,  551  (1898);  Trevor:  Jour. 
Phys.  Chem.,  3,  95  (1899). 

3  Monatsberichte  d.  Kgl.  Preuss.  Akad.,  Berlin,  1877. 

4  Zeit.  Phys.  Chem.,  4,  128  (1889). 

5  Wied.  Ann.,  40,  244  (1890). 

6  Zeit.  Phys.  Chem.,  5,  340  (1890). 


224668 


gams,  but  the  work  done  in  this  laboratory  has  made  it  pos- 
sible to  avoid  these  difficulties  so  that  the  present  investigation 
extends  over  the  greatest  range  possible,  the  limits  of  the  con- 
centration being  an  amalgam  saturated  with  zinc  on  the  one 
hand  and  an  amalgam  of  zero  concentration  or  pure  mercury 
on  the  other.  The  more  dilute  amalgams  in  this  range  had 
never  been  investigated  and  it  was  thought  very  likely  that 
dilutions  of  the  order  of  one  part  of  zinc  to  ten  million  or  one 
hundred  million  parts  of  mercury  would  give  some  very  inter- 
esting information  on  the  equilibrium  between  mercury  and 
a  zinc  sulphate  solution,  such  as  was  obtained  by  Hulett  and 
De  Lury  on  cadmium  amalgams. i  It  is  well  known  that  a  rod 
of  zinc  becomes  coated  with  mercury  when  placed  in  a  solution 
containing  a  mercury  salt.  If  this  reaction  is  reversible,  as  of 
course  it  must  be  if  our  ideas  of  chemical  equilibrium  are  cor- 
rect, we  should  expect  that  mercury,  when  put  into  a  zinc 
sulphate  solution,  would  precipitate  some  zinc  and  become 
a  very  dilute  amalgam.  This  expectation  was  substantiated 
by  the  experimental  results  as  will  be  shown  in  the  following 
pages. 

One  of  the  most  serious  difficulties  encountered  by  previous 
investigators  has  been  due  to  a  loss  of  zinc  from  the  amalgams 
by  oxidation  and  unless  this  difficulty  is  overcome  the  results 
are  not  reliable.  The  more  recent  workers  paid  a  great  deal 
of  attention  to  this  point,  but  they  were  not  in  a  position  to 
know  just  how  successful  their  precautions  were  since  they 
were  measuring  the  electromotive  force  of  these  amalgams  in 
a  concentration  cell  and  this  magnitude  depends  on  the  rela- 
tive and  not  on  the  absolute  concentrations  of  the  amalgams 
so  that  there  might  have  been  a  loss  of  zinc  from  all  of  the 
amalgams  in  the  cells  and,  provided  this  loss  in  each  case  was 
proportional  to  the  concentration,  there  would  have  been  no 
change  in  the  electromotive  forces  of  the  cells. 

In  the  present  work  we  employed  a  two-phase  reference 
electrode  in  each  cell.  This  electrode  was  of  such  a  nature 


Jour.  Amer.  Chem.  Soc.,  30,  1805  (1908). 


that  its  potential  was  independent  of  a  loss  or  gain  of  zinc 
but  depended  only  on  the  temperature,  therefore  it  was  ex- 
ceedingly reproducible  and  constant.  With  this  electrode  as 
a  basis  we  were  able  to  detect  at  any  time  a  loss  of  zinc  from 
any  of  the  unsaturated  amalgams  and  thus  we  were  enabled 
to  rigidly  control  each  step  of  the  experiment. 

The  difficulty  in  making  and  handling  the  amalgams  as 
well  as  the  problem  of  excluding  the  oxygen  has  been  over- 
come in  this  and  previous  work1  done  in  this  laboratory. 
Since  all  these  things  are  of  primary  importance  in  obtaining 
reliable  results,  the  apparatus  and  methods  will  be  described 
in  considerable  detail  in  this  thesis. 

Purification  of  Materials. — All  of  the  mercury  used  in  this 
investigation  was  chemically  purified  and  then  distilled  by 
the  method  devised  by  Hulett.2  In  this  method  a  continu- 
ous stream  of  air  is  allowed  to  bubble  through  the  mercury 
which  is  distilled  under  diminished  pressure.  The  air  is  essen- 
tial to  this  process  as  otherwise  both  zinc  and  cadmium  dis- 
til over  if  present. 

Mylius  and  Hromm3  have  succeeded  in  obtaining  very 
pure  zinc  and  their  most  satisfactory  method,  with  slight  mod- 
ifications, was  the  one  used  in  this  work.  The  zinc  sulphate 
was  first  dissolved  and  then  treated  with  Kahlbaum's  best 
zinc  oxide,  hydrogen  peroxide  and  electrolytic  ozone  being 
used  to  oxidize  any  ous  iron.  After  digesting  two  or  three 
days  on  a  water  bath  all  of  the  iron  was  considered  precipi- 
tated. The  solution  was  then  filtered,  made  slightly  alkaline 
with  ammonium  hydroxide  and  treated  with  hydrogen  sul- 
phide until  a  fairly  heavy  precipitate  of  zinc  sulphide  was 
obtained.  This  was  then  digested  for  two  or  three  days  on  a 
water  bath  with  frequent  agitation  which  insured  the  com- 
plete precipitation  of  the  heavy  metals.  The  solution  was 
then  filtered  and  the  zinc  sulphate  crystallized  out.  This 


1  Hulett  and  De  Lury:  Jour.  Amer.  Chem.  Soc.,  30,  1805  (1908). 

2  Phys.  Rev.,  21,  388  (1905). 

3  Zeit.  anorg.  Chem.,  9,  176  (1895). 


purified  zinc  sulphate  after  crystallizing  and  centrifuging  three 
times  was  used  as  the  electrolyte  from  which  our  electrolytic 
zinc  was  obtained.  The  anode  used  was  a  piece  of  platinum 
foil  of  about  6  cm2  area.  This  was  enclosed  in  a  filter  paper 
anode  cup.  In  order  to  keep  the  solution  from  becoming  acid 
during  the  electrolysis  the  anode  cup  was  kept  full  of  a  basic 
sulphate  of  zinc  prepared  in  the  following  manner:  ammo- 
nium hydroxide  was  added  to  a  solution  of  the  purified  zinc 
sulphate  until  the  precipitate  at  first  formed  redissolved.  This 
clear  solution  was  then  allowed  to  drop  slowly  into  boiling 
water,  a  steady  stream  of  steam  being  passed  in  to  prevent 
bumping.  A  white  crystalline  precipitate  was  obtained  that 
could  be  readily  filtered  and  washed.  The  cathode  was  a 
tenth  mm.  platinum  wire  sealed  into  a  small  glass  tube  so 
that  about  only  one-half  mm.  protruded.  The  current  was 
regulated  by  resistance,  the  density  being  kept  low  enough  to 
obtain  a  compact  deposit  of  zinc.  The  zinc  thus  obtained, 
after  being  washed  and  dried,  was  melted  in  a  hard  glass  tube 
with  ammonium  chloride  (specially  purified)  which  took  care 
of  the  dross  and  inclusions  and  left  the  zinc  clean  and  bright 
at  the  bottom.  This  ingot  was  carefully  scraped  with  clean 
steel  and  then  melted  in  a  high  vacuum  to  remove  any  volatile 
impurities.  It  was  then  transferred  to  another  hard  glass 
tube  and  distilled  in  vacuum.  The  distillation  was  always 
stopped  before  all  of  the  zinc  had  distilled  over.  The  dis- 
tilled portion  was  run  into  the  drawn  out  end  of  the  tube  and 
kept  sealed  up  until  used.  In  the  distillation  of  zinc  it  was 
found  necessary  to  continue  the  evacuation  until  the  zinc 
started  to  distil  over,  otherwise  all  the  gases  were  not  removed 
and  the  tube  softened  before  the  distillation  temperature  was 
reached. 

In  the  purification  all  cork  and  rubber  stoppers  and  con- 
nections were  carefully  avoided  and  when  connections  were 
necessary  the  glass  was  sealed  together.  The  solutions  were 
always  kept  in  Jena  flasks. 

Preparing  and  Handling  the  Amalgams. — The  masses  of 
mercury  and  zinc  necessary  for  making  an  amalgam  of  any 


given  concentration  can  be  very  accurately  weighed  out  on 
the  balance,  but  the  difficulty  of  mixing  the  two  metals  with- 
out a  loss  of  zinc  by  oxidation  has  been  a  very  serious  ob- 
stacle. This  has  been  overcome  by  the  following  very  sim- 
ple device  first  used  by  Hulett  and  De  Lury.1 

A  short  piece  of  glass  rod  (a)  Fig.  i  was  sealed  onto  the 


Fig.  i 

edge  of  a  small  crystallizing  dish.  The  glass  tube  (b)  had  a 
platinum  wire  sealed  into  its  lower  end  and  extending  up 
through  the  tube.  Another  platinum  wire  (c)  extended  across 
the  center  of  the  dish  and  was  sealed  onto  the  outside  of  the 
tube  (b).  The  tube  was  fastened  securely  to  the  upright  rod, 
(a),  by  means  of  insulating  tape  or  a  little  marine  glue  and 
thus  formed  a  very  compact  piece  of  apparatus.  The  mass  of 
zinc  to  be  used  in  a  given  amalgam  was  weighed  out  on  a  bal- 
ance sensitive  to  o.oi  mg.  The  mass  of  mercury  required  for 
a  given  concentration  was  then  calculated  and  weighed  out. 
All  weighings  were  made  by  substituting  calibrated  weights  for 
the  tared  metal.  The  vacuum  correction  was  applied  to  the 
weights  of  mercury  but  since  the  density  of  zinc  was  about  the 
same  as  that  of  the  weights  the  vacuum  correction  for  it  was 
negligible.  After  weighing,  the  mercury  was  put  in  the  ap- 
paratus just  described  (Fig.  i)  and  made  cathode  by  twelve 
volts  under  distilled  water.  The  weighed  pieces  of  zinc  were 


lyOC.     Clt. 


now  dropped  on  the  mercury,  and  dissolved  without  forming 
a  coat.  All  the  amalgams  made  up  in  this  way  presented  a 
surface  as  clean  and  bright  as  pure  mercury  and  could  be  pre- 
served for  any  length  of  time.  Amalgams  were  thus  made  up 
of  very  exact  composition  without  the  slightest  chance  of  a 
loss  of  zinc  by  oxidation.  It  has  been  suggested  that  the  mer- 
cury distilled  by  the  process  which  we  employed  might  con- 
tain dissolved  oxygen.1  There  is  no  evidence  of  the  solubility 
of  oxygen  in  mercury,  but  even  if  it  were  found  to  be  soluble 
there  is  no  question  that  the  e.  m.  f.  applied  in  the  above 
method  of  preparing  the  amalgam  would  rapidly  depolarize 
any  dissolved  oxygen.  It  also  reduces  any  coat  of  oxide  that 
might  have  formed  on  the  amalgam. 

Density  and  Composition  of  Zinc  Amalgams. — It  was 
desirable  to  know  the  concentration  of  zinc  in  the  liquid  por- 
tion of  the  standard  electrode  at  the  temperature  of  the  e.  m. 
f.  measurements,  25°.  The  work  of  Richards  and  Forbes2  on 
the  density  and  composition  of  zinc  amalgams  at  20°  indicated 
that  the  density  was  a  linear  function  of  the  composition,  but 
their  results  extended  only  over  a  limited  range.  We  have 
found  that  the  density,  at  25°  is  a  linear  function  of  the  com- 
position from  pure  mercury  up  to  the  saturated  amalgam. 
This  curve  was  accurately  located  so  that  it  was  only  neces- 
sary to  know  the  density  of  an  amalgam  to  calculate  its  com- 
position. 

The  pycnometer  used  (Fig.  2)  was  of  the  same  form  as 


Fig.  2 


1  T.  W.  Richards,  Carnegie  Pub.,  118,  p.  8. 

2  Zeit.  phys.  Chem.,  58,  693  (1907). 


that  used  by  Hulett  and  De  Lury1  which  was  found  to  have 
many  points  of  advantage  over  the  ordinary  type.  One  of 
the  capillary  tubes  had  a  single  mark  near  the  end  while  the 
other  was  marked  of!  into  twenty-one  equal  divisions.  This 
graduated  end  was  carefully  calibrated  so  that  it  was  only 
necessary  to  adjust  the  amalgam  to  the  single  mark  on  the 
other  end.  Each  division  on  this  stem  was  0.708  mm.  The 
capillary  was  filled  with  varying  amounts  of  mercury  and  the 
length  of  the  column  and  the  weight  were  carefully  determined. 
Three  determinations  gave  as  the  weight  of  one  division:  0.00230, 
0.00228  and  0.00231  respectively.  If  we  take  0.00230  as  the 
weight  of  mercury  required  to  fill  one  division  and  the  density 
of  mercury  at  25°  as  13.53396  the  volume  of  one  division  of 
the  stem  is  found  to  be  0.00017  cc. 

This  pycnometer  was  weighed  frequently  during  the  time 
it  was  used.  For  example  it  was  weighed  five  times  on  as 
many  successive  days,  three  of  these  weighings  were  exactly 
the  same  15.9621  the  other  two  were  15.9623  so  that  15.9622 
was  considered  very  near  the  true  weight.  The  average  of 
five  weighings  of  this  pycnometer  filled  with  mercury  to  the 
2ist  division  at  25°  gave  107.24385.  This  gives  91.26261  as 
the  mass  of  mercury  in  the  pycnometer  and  from  this  the  vol- 
ume at  25  is  calculated  to  be  6.7432  cc.  The  pycnometer  was 
then  filled  to  the  7th  division  of  the  capillary  and  found  to 
weigh  107.21124  which  gave  as  the  mass  of  mercury  91.23000 
giving  6.7408  as  the  volume  to  the  7th  division.  The  differ- 
ence of  these  two  volumes  0.0024  cc.  should  be  equal  to  the 
volume  of  the  fourteen  unfilled  divisions  of  the  capillary. 
The  volume  of  one  division  was  found  to  be  0.00017  so  that  the 
volume  of  fourteen  divisions  would  be  0.00238  showing  the 
two  results  to  be  in  perfect  agreement. 

The  densities  of  amalgams  of  four  different  concentra- 
tions were  determined  with  this  pycnometer.  The  amalgams 
containing  respectively  i,  il/2  and  2  grams  of  zinc  in  100 
grams  of  mercury  were  made  by  weighing  out  very  accurately 

1  Loc.  cit. 


10 

the  required  amounts  of  zinc  and  mercury  for  the  different 
concentrations.  The  zinc  was  weighed  out  first  on  a  balance 
sensitive  to  o.oi  mg.,  the  air  weight  of  mercury  required  was 
then  calculated  and  weighed  out.  By  means  of  a  fine  cap- 
illary joined  to  a  piece  of  rubber  tubing  the  amount  of  mer- 
cury was  easily  adjusted  to  one  or  two  tenths  of  a  milligram. 

Weight  of  zinc  1.48624  1.70912  3.12705 
Weight  of  mercury  148.63247  113.94540  156.85811 
Vacuum  cor rec  tion  o .  008 17  o .  006  26  o .  0086  2 
Mass  of  mercury  148.62430  113.93914  156.84949 
Grams  of  zinc  per 
100  grams  of  mer- 
cury 0.999998  1.500028  1.993600 

The  saturated  amalgam  was  made  by  rotating  an  excess 
of  zinc  with  mercury  in  a  25°  thermostat.  The  separation  of 
the  saturated  amalgam  from  the  excess  of  zinc  without  chang- 
ing the  temperature  was  effectively  accomplished  by  the  use 
of  the  piece  of  apparatus  shown  in  Fig.  3.  The  mercury  with 


> 
>=* 


Fig.  3 

an  excess  of  zinc  was  placed  in  (A)  through  the  end  (a)  which 
was  then  sealed  off.  The  cock  (C)  was  closed  and  an  aspira- 
tor applied  to  (b).  After  the  pressure  in  B  had  been  reduced 
to  about  3  cm.  the  thickened  part  at  (b)  was  sealed  off.  This 
whole  apparatus  was  then  slipped  into  a  large  glass  tube  and 
rotated  at  25°  ±  0.02  for  several  weeks,  then  without  removing 
it  from  the  bath  the  apparatus  was  held  in  a  vertical  position, 
the  end  (E)  being  down,  and  after  standing  one  hour  the  cock 
(C)  was  slowly  turned,  the  amalgam  then  passed  into  (B) 
through  the  filter  (D),  composed  of  asbestos  well  packed  in. 
Since  zinc  floats  on  the  amalgam  and  the  amalgam  is  drawn 
off  from  the  bottom  there  can  be  little  doubt  that  this  method 
effectively  separated  the  two  phases.  As  soon  as  sufficient  of 
the  amalgam  had  passed  into  (B)  the  apparatus  was  removed 


II 


from  the  bath,  the  tube  (B)  cut  open  and  the  amalgam  brought 
into  the  apparatus  (Fig.  i)  and  made  cathode  by  12  volts 
under  distilled  water  until  used. 

To  insure  complete  saturation  the  zinc  and  mercury  were 
usually  heated  up  to  50°  or  100°  with  frequent  agitation  before 
the  rotation  began  so  that  the  equilibrium  was  approached 
from  the  higher  temperature. 


Composition 

Pycnometer  +  amalgam 
Pycnometer 
Weight  of  amalgam 
Vacuum  correction 
Mass  of  amalgam 

Volume  of  pycnometer 
Density 


DENSITY  OF  THE:  SATURATED  AMALGAM. 


DENSITIES. 

I/IOO 

1.5/100 

1.9936/100 

106.66306 

i  06  .  34468 

106.04772 

15.97622 

15.97622 

15.97622 

90.68764 

90  .  36846 

90.07150 

o  .  00498 

o  .  00497 

o  .  00495 

90.67286 

90  .  36349 

90.06655 

(to  i8th) 

(to  7th) 

(to  3rd) 

6.7427 

6  .  7408 

6  .  7401 

13.4490 

13.4054 

13.3628 

From  three  tubes 
Pycnometer  +  amalgam 
Pycnometer 
Weight  of  amalgam 
Vacuum  correction  ' 
Mass  of  amalgam 
Pycnometer  division 
Volume  of  pycnometer 
Density 
Average 


A 

105.95668 

15.97622 

89.98046 

o . 00495 

89-9755I 
(to  2oth) 

6 • 7430 
13-3435 


B 

105-94731 
15.97622 
89.97109 
o . 00495 
89.96614 
(to  i 7th) 
6-7425 
I3-343I 
13-34333 


105.95207 
15.97622 
89-97585 

o . 00495 

89.97090 

(to  i 8th) 

6.7427 

13-3434 


Taking  the  density  of  pure  mercury  at  25°,  (13.5340)  and 
that  of  i,  iV2,  and  2  percent  amalgams  the  density  composi- 
tion curve  was  plotted.  The  following  relation  is  the  result  of 
the  above  determinations  and  represents  very  accurately 
the  density  of  any  amalgam.  D25o  =  13.5340  —  0.0859  p 
where  p  is  the  number  of  grams  of  zinc  in  100  grams  of  mer- 
cury. The  point  on  this  line  which  corresponds  to  the  density 
of  the  saturated  amalgam,  13.34333,  indicates  a  composition 
of  2.2196  grams  of  zinc  in  100  grams  of  mercury. 

The  composition  of  the  saturated  amalgam  prepared  as 
described  above  was  also  determined  by  analysis.  The  amal- 


12 

gam  was  treated  with  hydrochloric  acid1  (i  vol.  cone,  acid  to 
i  vol.  of  water)  which  dissolved  the  zinc  with  an  energetic 
evolution  of  hydrogen  and  never  dissolved  a  weighable  amount 
of  mercury  unless  the  acid  was  allowed  to  stand  in  contact 
with  the  mercury  many  times  longer  than  was  necessary  for 
the  removal  of  the  zinc.  It  was  found  necessary  to  hasten 
the  reaction  by  means  of  a  platinum  spiral  which  made  con- 
tact with  the  amalgam  and  extended  up  through  the  acid. 
A  small  glass  bead  was  fused  on  the  wire  just  above  the  point 
reached  by  the  amalgam  surface  to  prevent  the  mercury  from 
creeping  up  the  wire  and  destroying  its  effectiveness.  After 
all  of  the  zinc  had  been  dissolved  the  acid  was  removed  with 
a  pipette  and  the  mercury  washed  and  dried  in  a  vacuum 
desiccator  over  calcium  chloride.  It  was  found  necessary  to 
use  every  precaution  to  prevent  a  loss  of  mercury  by  ''spurt- 
ing" when  the  desiccator  was  evacuated.  In  order  to  dry 
the  mercury  effectively  a  comparatively  high  vacuum  was 
necessary  and  if  any  particles  of  water  happened  to  be  under 
the  mercury  their  rapid  expansion  threw  a  fine  spray  of  mer- 
cury up  against  the  sides  of  the  flask  and  out  of  the  top  if 
uncovered.  As  much  as  twenty  milligrams  of  mercury  were 
often  lost  in  this  way.  This  difficulty  was  effectively  over- 
come by  providing  the  analysis  flasks  with  loosely  fitting 
stoppers,  setting  them  in  shallow  dishes  and  covering  them 
with  inverted  beakers  when  they  were  put  in  the  desiccator. 
The  following  are  the  results  of  duplicate  analysis  of  amalgams 
taken  from  two  different  tubes  I  and  II. 

i.  II. 


Flask  and  amalgam 

23 

.9132 

28 

.I76l 

35 

.0044 

5I-0937 

After  treatment  with 

acid  23 

.5386 

27 

.8001 

34 

3882 

50.2204 

Mass  of  zinc 

o 

3746 

0 

.3760 

o 

6l62 

0-8733 

Flask 

6 

6599 

IO 

.8565 

6, 

6600 

10.8570 

Weight  of  mercury 

16 

.8787 

16 

9436 

27. 

7282 

39-3634 

Mass  of  mercury 

16 

,8778 

16 

.9427 

27, 

7267 

39-36I3 

Grams  of  zinc  in  100 

2 

.2194 

2 

.  2192 

2 

.  2224 

2.2186 

Average  2.2193  2.2205 


1  Kerp  and  Bottger:  Zeit.  anorg.  Chem.,  25,  i  (1906). 


13 

The  average  of  these  two  values  gives  2.2199  as  the  num- 
ber of  grams  of  zinc  soluble  in  100  grams  of  mercury  at  25°. 
This  value  is  in  good  agreement  with  the  value  2.2196  calcu- 
lated from  the  density-composition  relations.  These  results 
are  very  probably  reliable  to  the  third  place  decimal  so  that 
2. 2 20  has  been  taken  as  the  correct  value. 

The  Coefficient  of  Expansion  of  Zinc  Amalgams  has  also 
been  measured.  The  dilatometer  used  consisted  of  a  glass 
tube  of  about  8  cm.  internal  diameter  and  about  80  cm,  long, 
to  one  end  of  which  was  sealed  a  capillary  stopcock  and  to  the 
other  a  capillary  tube  about  35  cm.  long.  A  thermometer 
scale  was  fixed  to  this  capillary  tube  and  the  instrument  care- 
fully calibrated  with  mercury.  The  weights  of  mercury 
required  to  fill  it  to  various  divisions  of  the  scale  at  25°  were 
accurately  determined  and  the  volume  thus  calculated. 

Division  on  scale  at  25°  I29-3  249-5 

Air  wt.  of  mercury  86.1477  86.3226 

Mass  of  mercury  81 . 1430  86.3179 

Volume                                                        6.36500.  6.37800. 

The  difference  of  these  two  volumes,  0.013  cc.,  should  be 
equal  to  the  volume  of  (249.5  -  I29-3)>  120.2  divisions  of  the 
capillary  stem. 

This  capillary  was  calibrated  separately : 

501 . 5  divisions  of  mercury  weighed  0.8002  g.  wt.  of  rdiv.  =  0.001425 
328  .o  divisions  of  mercury  weighed  0.4673  g.  wt.  of  i  div.  =  0.001425 

From  this,  the  volume  of  one  division  is  found  to  be  0.000105 
cc.  and  the  volume  of  120.2  divisions  is  0.0126  cc.,  which  is  in 
fair  agreement  with  the  value  of  0.013  obtained  above.  The 
dilatometer  was  now  filled  with  mercury  to  the  224th  division 
when  at  25°;  it  was  then  put  in  a  35°  thermostat  and  the 
mercury  in  the  capillary  rose  to  317.2  on  the  scale.  We  may, 
within  the  error  of  the  experiment,  consider  0.000181  as  the 
coefficient  of  cubical  expansion  of  mercury.  From  this  and 
the  above  data,  the  coefficient  of  cubical  expansion  of  the 
glass  is  found  to  be  0.0000279,  which  is  about  the  accepted 
value  for  the  kind  of  glass  used.  A  1 1/2  percent  amalgam  was 
put  in  the  dilatometer: 


At  25.01°  the  amalgam  reached  to  260.5  on  the  scale. 
At  35-03°  tne  amalgam  reached  to  349.0  on  the  scale. 

For  a  rise  of  10.02°  there  was  an  increase  of  88.5  divisions  or 
0.00929  cc.  when  the  volume  at  the  lower  temperature  was 
6.379  cc.  Taking  into  account  the  expansion  of  the  glass  as 
determined  above,  we  get  0.000173  as  the  coefficient  of  cubical 

expansion  of  a  -    -  amalgam. 

ji/ 
The  density  of  a  -  -  zinc  amalgam   at  25°  was  found  to 


I  OO 


be  13.4054.     The  density  at  20°,  calculated  by  means  of  the 

above  coefficient,  is,  D20  =  13.4171,  giv- 

i  —  (0.000173  X  5) 

ing  a  change  of  0.0117  in  density  for  a  change  of  5°.  The 
change  in  density  for  pure  mercury  for  the  same  range  in 
temperature  is  0.0122.  The  difference  of  0.0005  in  the  two 
values  is  due  to  the  presence  of  il/2  grams  of  zinc  in  100  grams 
of  mercury.  From  this  the  difference  which  would  be  caused 
by  i  gram  of  zinc  in  100  grams  of  mercury  for  a  temperature 
change  of  i°  is  calculated  to  be  0.000067. 

The  density  of  an  amalgam  at  any  temperature  is  given 
by  the  relation 

D,  =  =  D25  —  (0.00244  —  0.000067  p)  (t  —  25) 
where  D25  =  density  at  25°,  p  •-     grams  of  zinc  in  100  grams 
of  mercury,  and  t  --=  temperature;  or,  combining  this  relation 
with  the  expression  for  the  density  of  any  amalgam  at  25°, 
we  have: 
D<  =--  I3-5340  —  0.0859  />  —  (0.00244  —  0.000067  P)  (*  —  25). 

Since  the  density  is  not  a  linear  function  of  the 
temperature  this  equation  applies  only  over  a  limited  range 
of  temperature. 

Electromotive  Force  of  Zinc  Amalgams 

No  reliable  observations  could  be  obtained  on  the  e.  m.  f. 
of  dilute  amalgams  until  means  were  devised  for  really  remov- 
ing and  excluding  oxygen.  This  is  fundamental  for  all  work 
with  dilute  amalgams.  For  this  purpose  it  was  very  impor- 


15 

tant  to  have  an  adequate  supply  of  hydrogen  which  was 
unquestionably  free  from  oxygen. 

The  Preparation  of  Oxygen-Free  Hydrogen. — The  hydro- 
gen was  generated  in  a  Kipp  generator  from  Kahlbaum's  No. 
II  zinc  and  Baker's  analyzed  hydrochloric  acid.  The  gas  after 
passing  through  a  tower  containing  soda  lime  and  calcium 
chloride  was  conducted  over  a  platinum  spiral  consisting  of 
over  a  meter  of  tenth  mm.  platinum  wire  wound  on  a  small 
porcelain  tube.  When  hydrogen  was  being  used  this  plati- 
num wire  was  glowed  continuously  with  an  electric  current 
and  caused  the  hydrogen  to  combine  with  any  residual  oxy- 
gen. Nernst1  and  his  co-workers  have  shown  that  in  cases  of 
this  kind  the  gaseous  mixtiire  very  rapidly  comes  to  the  equilib- 
rium conditions  corresponding  to  the  temperature  of  the  wire. 
They  have  also  given  us  the  necessary  data  for  calculating 
the  equilibrium  constant  for  the  equation  2H2O  ~^~>'  2H2  +  O2 
for  any  given  temperature.  The  temperature  of  the  spiral 
was  not  far  from  1000°  absolute  so  that  the  calculated  partial 
pressure  of  the  oxygen  in  our  hydrogen  was  of  the  order  of 
magnitude  of  4  X  io~l6  mm.  Oxygen  then  was  practically 
out  of  the  gas  phase.  All  hydrogen  passed  over  this  glowing 
spiral  immediately  before  it  was  used. 

Cell. — The  form  of  cell  used  is  shown  in  Fig.   4.     The 


Fig.  4 

cell  was  separated  into  compartments,  by  little  dams  about  i 


1  Thermodynamics  and  Chemistry;  Nernst.  Langmuir:  Jour.  Am.  Chem. 
Soc.,  28,  1357  (1906). 


i6 

cm.  high  which  prevented  the  mixing  of  the  amalgams  but 
allowed  a  thorough  mixing  of  the  electrolyte  when  the  cell 
was  rocked.  The  contact  wires  were  short  l/  10  mm.  platinum 
wires  sealed  into  tubes  which  made  well  ground  joints  with  the 
tubes  a,  b,  c,  d  and  e.  The  application  of  a  very  little  marine 
glue  made  these  joints  perfectly  tight.  A  little  mercury  was 
introduced  into  these  tubes  carrying  the  contact  wires  and 
made  contact  between  the  platinum  wires  and  the  copper 
wires  which  extended  out  of  the  cell.  A  spiral  of  l/20  mm. 
platinum  wire  (g)  was  fixed  inside  the  cell  by  means  of  two 
larger  lead  wires  sealed  through  the  sides. 

The  side  tube  was  connected  by  means  of  a  T  tube  and 
two  stopcocks  to  the  hydrogen  generator  and  a  vacuum  pump. 
The  cell  was  first  evacuated  and  then  hydrogen  was  let  in, 
this  hydrogen  with  the  residual  oxygen  was  then  removed  by 
the  vacuum  pump  and  more  hydrogen  let  in.  This  process 
was  repeated  until  the  air  originally  in  the  cell  was  replaced 
by  hydrogen.  The  platinum  wire  (g)  was  then  glowed  to 
cause  any  residual  oxygen  to  combine  with  hydrogen. 

The  Electrolyte. — The  electrolyte  was  placed  in  a  separa- 
tory  funnel,  the  stem  of  which  was  connected,  in  the  same 
manner  as  the  cell  was,  with  a  vacuum  pump  and  the  hydro- 
gen generator.  The  separatory  funnel  was  first  evacuated 
and  the  solution  shaken  vigorously  in  vacuum,  hydrogen  was 
then  admitted  and  the  solution  shaken  again  to  saturate  it 
with  hydrogen,  this  hydrogen  with  the  residual  oxygen  was 
removed  by  evacuation  and  shaking  and  fresh  hydrogen  again 
let  in.  This  process  was  repeated  six  or  seven  times  before 
the  electrolyte  was  put  into  the  cell.  One  of  the  ground  glass 
joints  was  then  opened,  after  the  marine  glue  had  been  warmed, 
and  the  oxygen  free  electrolyte  was  introduced  against  a  rapid 
stream  of  hydrogen.  The  other  materials  which  were  intro- 
duced into  the  cells  were  also  well  washed  by  the  hydrogen 
which  was  forced  out  through  the  tube  through  which  they 
were  introduced.  The  spiral  (g)  was  continually  glowing  and 
took  care  of  any  oxygen  which  might  have  escaped  these  pre- 
cautions. 


The  electrolyte  in  some  cases  was  a  saturated  solution  of 
zinc  sulphate  with  an  excess  of  zinc  sulphate  crystals  cover- 
ing the  amalgams,  in  other  cases  a  solution  which  was  not 
quite  saturated  at  25°  was  used. 

The  amalgams  up  to  a  dilution  of  — -  (that  is  one  part  of 

zinc  to  10,000  parts  of  mercury)  were  made  as  follows.  The 
zinc  was  cut  into  small  pieces  so  that  there  was  no  especial 
difficulty  in  obtaining  the  correct  mass  of  zinc  for  just  enough 
of  the  given  amalgam  for  one  of  the  compartments  of  the  cell. 
The  mass  of  this  zinc  was  determined  to  o.oi  mg.  The  air 
weight  of  mercury  was  then  calculated  and  this  amount  of 
weights  was  placed  on  the  balance  pan  together  with  a  tube 
of  the  form  shown  in  Fig.  5.  The  diameter  of  the  largest  part 


of  the  tube  was  about  15  mm.  The  end  (a)  was  drawn  down 
to  about  5  mm.  and  (b)  to  about  0.3  mm.  internal  diameter. 
The  end  (c)  was  closed  and  bent  as  shown.  Just  at  the  bend  a 
scratch  was  made  with  a  diamond.  On  the  balance  this  tube 
rested  on  a  cork  support  (d).  After  this  tube  and  the  weights 
were  tared  the  weights  were  removed  and  the  correct  amount  of 
mercury  put  into  the  tube,  the  last  bit  being  adjusted  with  a 
capillary.  The  tube  was  now  removed  from  the  balance  and 
(a)  was  connected  by  means  of  two  stopcocks  alternately  with 
an  aspirator  and  the  hydrogen  generator  until  the  air  about 
the  mercury  had  been  replaced  with  hydrogen. 

One  of  the  compartments  of  the  cell  was  now  opened  and 


i8 

the  end  (c)  of  the  tube  slowly  introduced  and  thus  washed 
externally  with  the  stream  of  hydrogen  which  was  forced  in 
through  the  side  tube  and  out  through  the  opened  tube  (Fig.  4) . 
By  gently  pressing  the  end  of  the  tube  (b)  on  the  bottom  of 
the  cell,  the  end  (c)  was  broken  of!  at  the  point  where  it  had 
been  scratched  and  the  mercury  flowed  down  gently  into  the 
compartment  under  pressure  of  hydrogen  from  the  generator. 

The  pieces  of  zinc  were  now  held  for  some  seconds  in  the 
stream  of  hydrogen  issuing  from  the  cell  and  then  lowered 
slowly  about  half  way  down  the  tube  to  wash  them  with 
hydrogen;  finally  they  were  dropped  into  the  mercury  where 
they  soon  dissolved.  This  method  could  not  be  used  for  pre- 
paring amalgams  more  dilute  than  one  part  of  zinc  to  ten  thou- 
sand parts  of  mercury  on  account  of  the  difficulty  of  weigh- 
ing such  a  small  amount  of  zinc  with  the  required  accuracy. 

By  comparing  the  potential  of  an  amalgam  made  in  this 
way  with  one  of  the  same  concentration  made  up  outside  of 
the  cell  and  then  introduced  we  have  found  that  the  method 
of  making  up  the  amalgam  in  the  cell  is  the  more  reliable  of 
the  two,  since  it  is  almost  impossible  to  introduce  a  very  dilute 
amalgam  without  some  oxidation  taking  place. 

In  the  case  of  the  more  dilute  amalgams  the  electrolyte 
and  the  two-phase  amalgam  were  introduced  in  the  manner 
just  described;  weighed  amounts  of  mercury  were  put  into  the 
other  compartments  and  after  the  ground  joints  were  all 
sealed  with  marine  glue  the  side  tube  was  sealed  off  with  a 
blowpipe.  The  wire  (g)  in  the  cell  was  brought  to  a  glow 
with  an  electric  current  intermittently  for  three  or  four  days, 
the  contents  of  the  cell  being  agitated  continuously  during 
this  time.  This  was  effected  by  an  eccentric  attached  to  a 
water  motor  which  slowly  raised  and  lowered  one  end  of  the 
cell  causing  the  electrolyte  to  flow  back  and  forth  over  the  dams. 
The  wire  was  glowed  every  few  seconds  instead  of  continu- 
ously because  by  this  means  less  heat  was  generated  in  the 
cell. 

As  has  been  shown  (p.  15)  the  glowing  removed  any 
residual  oxygen  in  the  gas  phase  and  it  seems  that  any  trace 


19 

of  dissolved  oxygen  in  the  electrolyte  would  have  been  com- 
pletely removed  by  the  agitation  in  an  atmosphere  of  pure 
hydrogen  and  continually  coming  into  contact  with  the  two- 
phase  amalgam.  The  cell  was  then  put  in  the  25°  bath  and 
zinc  from  the  two-phase  amalgam  was  deposited  electrolytic- 
ally  in  the  mercury.  These  two  methods  of  making  up  the 
amalgams  were  made  to  overlap.  That  is  two  amalgams  con- 
taining one  part  of  zinc  to  ten  thousand  parts  of  mercury  were 
made  up,  one  by  weighing  out  the  metals  directly  and  intro- 
ducing them  separately  into  the  cell,  the  other  by  depositing 
the  zinc  electrolytically,  and  the  potentials  of  the  two  were 
in  excellent  agreement. 

Measurement  of  Electromotive  Forces. — The  basis  of  all 
e.  m.  f.  measurements  was  Clark  cells  which  had  been  made 
at  intervals  since  1903  and  all  of  which  were  in  good  agree- 
ment. These  cells  are  reproducible  to  better  than  one  part 
in  50,000,  their  value  was  taken  to  be  1.42040  volts  at  25 °.1 
For  this  work  a  loo-liter  oil  bath  was  used.  It  was  elec- 
trically heated  and  controlled  and  did  not  vary  from  25° 
by  more  than  0.01°  at  any  time.  The  thermometer  used  in 
locating  this  temperature  was  checked  at  the  transition  point 
of  Glauber's  salt,  32.383°. 

The  instruments  used  in  the  e.  m.  f.  measurements  were 
a  high  resistance  Wolf!  potentiometer,  a  Broca  glavanometer 
with  1000  ohm  coils;  a  Nernst  glower  lamp  and  scale,  a  reg- 
ulating resistance  and  two  standard  batteries2  which  gave  a 
very  steady  and  constant  current.  The  resistance  coils  of 
the  potentiometer  were  carefully  calibrated  and  the  whole 
system  thoroughly  insulated.  Moisture  or  dust  on  the  hard 
rubber  base  of  the  potentiometer  introduced  very  consider- 
able disturbances  so  the  instrument  was  kept  scrupulously 
clean  and  on  damp  days  no  measurements  were  taken. 

The  Standard  Electrode. — Previous  workers  have  observed 
that  the  e.  m.  f.  between  two  dilute  zinc  amalgams  of  different 
concentrations  increases  considerably  with  the  time.  It  is 

1  Phys.  Rev.,  32,  275. 

2  Phys.  Rev.,  27,  33  (1908). 


20 

obvious  that  this  increase  did  not  show  the  actual  loss  of  zinc 
but  only  the  relatively  greater  loss  of  the  more  dilute  amal- 
gam, since  the  e.  m.  f.  depends  only  on  the  ratio  of  the  two 

RT      C 

concentrations  according  to  the  equation,  e.  m.  f.  =  -— ,  In-^, 

nb        C2 

and  if  both  amalgams  lost  zinc  in  proportion  to  their 
concentration  there  would  be  no  change  in  e.  m.  f.  This  fact 
was  pointed  out  by  Hulett  and  De  Lury1  in  their  work  on 
cadmium  amalgams  and  they  made  use  of  a  two-phase  amal- 
gam as  a  standard  basis  of  reference  for  all  potential  measure- 
ments, since  the  potential  of  such  an  amalgam  depends  only 
on  the  temperature.  This  principle  has  been  made  use  of  in 
the  present  work  and  an  amalgam  containing  about  5  percent 
of  zinc  has  been  used  as  the  basis  of  all  the  potential  measure- 
ments. It  is  well  known  that  for  a  given  temperature  the 
potential  of  all  zinc  amalgams  containing  something  over 
2  percent  of  zinc  is  constant.  Considerable  loss  of  zinc  by 
oxidation  or  from  any  cause  could  not,  therefore  affect  the 
concentration  or  the  potential  of  the  standard  electrode,  so 
that  any  increase  in  e.  m.  f.  between  this  standard  and  a  dilute 
amalgam  indicated  a  loss  of  zinc  from  the  dilute  amalgam 
only  and  the  exact  amount  of  this  loss  could  be  easily  calcu- 
lated. 

The  cells  used  for  the  amalgams  had  five  or  six  compart- 
ments and  each  amalgam  was  measured  against  the  constant 
electrode.  The  e.  m.  f.  between  any  two  amalgams  was  given 
then  by  the  difference  between  e.  m.  f.'s  as  measured  against 
the  constant  electrode.  This  method  of  measurement  is  the 
only  one  as  yet  employed  by  which  one  can  be  sure  as  to 
whether  any  amalgam  is  losing  zinc.  The  values  obtained  by 
taking  the  difference  between  the  e.  m.  f.'s  of  two  amalgams 
against  the  constant  electrode  and  the  value  obtained  by 
direct  measurement  were  compared  and  never  differed  by 
more  than  a  microvolt.  The  reproducibility  of  the  constant 
electrode  is  shown  by  the  following  measurements.  Two 
amalgams  containing  respectively  5  parts  and  2.5  parts  of 

1  Loc.  cit. 


21 


zinc  to  100  parts  of  mercury  were  put  into  two  compartments 
of  a  cell.  At  30°  or  below,  these  two  amalgams  should  have 
the  same  potential,  as  solid  zinc  is  present  in  both  amalgams 
at  or  below  this  temperature. 


Mar. 


25°  e.  m.  f.  between  the  two  was  0.000008  volt. 


Mar.  25th,  20°  e.  m.  f.  between  the  two  was  0.000006  volt. 
Mar.  3oth,  15°  e.  m.  f.  between  the  two  was  0.000003  v°lt- 


Apr.  iQth,  25°  e.  m.  f.  between  the  two  was  0.000002  volt. 
May     6th,  30°  e.  m.  f.  between  the  two  was  o.oooooo  volt. 

Another  of  the  advantages  in  measuring  all  potentials 
against  the  standard  electrode  is  that  the  results  from  any 
two  cells  are  directly  comparable  since  the  e.  m.  f  .  of  any  amal- 
gam concentration  cell  is  independent  of  the  concentration 
of  the  electrolyte.  It  is  only  necessary  to  accurately  control 
the  temperature. 

It  was  found  convenient  to  express  the  concentrations  as 
fractions  the  numerator  giving  the  grams  of  zinc  and  the  de- 
nominator the  grams  of  mercury.  The  concentration  of  the 


standard  electrode  is  then 


2. 220 
100 


at  25°. 


Cells  charged  with  amalgams,  gave  as  follows 

No.  i. 

At  25° 


Days 

I 

2 

Put  at  30° 

5 
At  30° 

i 

2 

3 

At  20° 
i 

2 

3 
At  15° 

i 

4 
6 


b 

2.5 
100 


2  phase 


100 
0.000878 
0.000875 


d 

i 

too 

0.007780 
0.007782 


1000 

0.035470 
0.035470 


0.000005  0.000887  0.007787  0.035520 


0.0000  10 

o  .  000005 

O.OOOOIO 

o  .  000003 
o  .  oooooo 

0.001670 
0.001722 
0.001728 

o  .  000047 
o  .  000040 
0.000047 

0.000015 
o  .  000003 

0  .  000002 

0.008712 
0.008771 
0.008777 

o  .  006800 
0.006796 
o  .  006800 

0.005797 
0.005782 
0.005784 

0.036920 
0.036961 
0.036964 

o  .  034064 

0.034057 
0.034075 

0.032585 
0.032579 
0.032589 

No.  2. 


Ato' 


Days 

> 

I 
2 

4 
10 
ii 

At  30.02° 
i 

5 
6 

.  o 


2.5 

IOO 


2  phase 


2 
IOO 


I 
IOO 


1000 


o.oooooi  0.000009 


0.000005  0.001740 

o.oooooo  0.001731 

0.001734 


0.002745  0.028110 

0.002730  0.028195 

0.002660  0.028054 

0.002551  0.027892 

0.002536  0.027881 

0.008796  0.037160 

0.008794  0.037180 

0.008794  0.037190 


At  35' 


At  25 


i 

2 

4 

10 

12 

o 

2 

6 


Days        2  phase 


2.2 

IOO 


0.000668 
o . 000845 

0.000774 
0.00o6l8 
o . 0006 i 5 

O . 000002 
O.OOOOOI 

No.  3. 

2 
IOO 


0.002680 
0.002861 
0.002800 
0.002630 
0.002635 

0.000892 
0.000887 


IOO 


0.009883 
0.010070 
0.010017 
o . 009846 
0.009861 

o . 007800 
0.007792 


IOO 


0.038768 
0.038980 

0.038945 
0.038815 
0.038800 

0.035614 
0.035609 


IOO 


At  25° 

i 

3 

4 
At  30° 

i 

4 
5 

Back  at  25 
5 


0.000068  0.000675  0.001640  o 
0.000074  0.000875  0.001829  o 
0.000047  0.000873  0.001829  o 

O.OOI368  O.OO2222  O.OO32OO  O 

0.000850  0.001712  0.002680  o 
0.000862  0.001720  0.002694  o 


004060  0.015432 

004253  0.015620 

004260  0.015625 

005680  O.OI7255 

005172  0.016759 

005183  0.016760 


O.OOOO6O  O.OOO88O  O.OOI83I  O.OO4262  0.015650 


Solubility  of  Zinc  in  Mercury. — The  e.  m.  fs.  between  the 
two-phase  electrode  and  amalgams  varying  in  concentration 
from  i  /ioo  to  2.5/100  have  been  plotted  against  the  tempera- 
ture in  Fig.  6.  The  points  for  any  one  amalgam  lie  in  a  straight 
line,  showing  that  the  temperature  coefficient  is  constant  for 
this  range  of  temperature.  The  points  at  which  these  lines 
cut  the  line  of  zero  potential  should  represent  the  tempera- 


23 


tures  at  which  these  amalgams  become  saturated.  By  inter- 
polation the  solubility  of  zinc  in  mercury  at  any  temperature 
within  the  range  may  be  calculated.  For  example,  it  is  seen 
that  a  line  which  would  cut  the  line  of  zero  potential  at  25° 
would  represent  an  amalgam  of  somewhat  greater  concentra- 
tion than  2.2/100.  At  this  point  the  decrease  in  e.  m.  f.  with 
an  increase  in  concentration  is  fairly  uniform,  an  increase  of 
o.i/ 100  in  concentration  representing  a  decrease  of  about 
0.000395,  in  e.  m.  f.  The  e.  m.  f.  of  the  2/100  amalgam  at  25° 
is  0.000875 ;  dividing  this  by  0.000395  we  find  that  an  increase  of 


rooo 

606C 


3000 
2000 


0° 


1.87° 


10° 


14.41 


19.71° 

20° 

Temperature 
Fig.  6 


o. 221/100  in  concentration  would  cause  the  amalgam  to  give 
zero  potential,  that  is,  the  saturated  amalgam  at  25°  con- 
tains 2.221  grams  of  zinc  to  100  grams  of  mercury.  This 
value  is  in  good  agreement  with  the  average  value,  2.220 
(page  13),  obtained  from  the  density  determinations  and  from 
the  analyses.  We  have  then  three  independent  methods  of 
determining  the  composition  of  a  zinc  amalgam. 

The  temperatures  at  which  the  lines  on  Fig.  6  cut  the  line 
of  zero  potential  are  given  below : 

Temperature  at  which  line 


Amalgam 
1.4 
1.8 

2.0 


cuts  line  of  zero  potential 

1.87° 
14.41 
19.71 


24 

That  is,  at  1.87°,  100  grams  of  Hg  should  dissolve  1.4 
grams  of  zinc,  etc. 

Saturated  amalgams  were  made  up,  by  the  method  already 
described,  (page  10)  at  o°,  15°  and  30°,  and  analyzed. 


30' 


ABABA  B 

1.387  1.388  1-832  1.827  2.431  2.432 

Average  1.3875  1.830  2.4315 

From  these  three  results  and  the  one  already  obtained 
for  25°,  the  solubility  curve  is  plotted  and  shown  in  Fig.  7. 


3 

f 

4 
V 

fi 

1     * 
I 

fc 
O 

1#V- 

Jl 

\° 

00° 

\p 

&* 

^ 

o^ 

^ 

^s- 

^ 

\* 

#>* 

**" 

^ 

*»^* 

* 

^ 

^ 

,  —  —  • 

^^ 

^^*" 

fc        < 

a 

I: 

oc 

10°                                      20°                                       30° 

Temperature 
Fig.  7. 

The  points  marked  X  are  obtained  from  the  intersections  on 
Fig.  6  as  already  explained.  The  solubility  of  zinc  in  mer- 
cury may  be  very  accurately  expressed  by  the  relation 

X     --  1.388  +  0.0242  t  +  0.000352  t2, 
where  X  represents  the  number  of  grams  of  zinc  in  100  grams 


25 

of  mercury,  and  t  the  temperature.  B.  Cohen  and  Inouye1 
have  recently  determined  the  solubility  of  zinc  in  mercury 
from  o°  to  100°  and  their  results  up  to  50°  are  represented  by 
the  above  equation.  For  temperatures  above  50°  a  term 
containing  t3  would  have  to  be  added. 

The  Temperature  Coefficient  of  Zinc  Amalgam  Cells.  —  As 
shown  in  the  tables,  the  e.  m.  f.  of  the  concentrated  amalgams 
was  measured  at  o°,  15°,  20°,  25°,  30°  and  35°.  At  25°  the 
temperature  of  the  thermostat  did  not  vary  more  than  a  few 
thousandths;  at  the  other  temperatures,  not  more  than  0.01°. 
In  order  to  keep  the  temperature  constant  at  the  higher  tem- 
peratures it  was  found  necessary  to  cover  the  thermostat  with 
a  box  made  of  asbestos  board.  A  small  incandescent  bulb 
kept  the  temperature  of  the  air  within  this  box  a  few  degrees 
below  that  of  the  thermostat.  Two  thermometers  calibrated 
by  the  Reichsanstalt  were  used  in  determining  the  tempera- 
tures. These  thermometers  were  checked  at  the  transition 
point  of  Glauber  salt  32.383°. 

The  value  of  ^~  of  each  amalgam  measured  against  the 
constant  electrode  is  given  below: 

Concentration  -^-  Diffs. 

I?100      0.0001680    0.0000046 

I'8/100  0.0001726 


1.4/100          0.0001842 
i  .0/100         o.oooI975 

0-5/100  0.0002270 

The  temperature  coefficient  for  any  pair  of  amalgams  may 
be  obtained  by  taking  the  difference  between  their  tempera- 
ture coefficients  as  measured  against  the  constant  electrode. 

For    example,    between    amalgams    2/100    and   i/ioo   -^  = 

0.00002  95;  between  1.8/100  and  i/ioo  ^  =  00.0000249,  etc. 

As    is  well   known,  the    e.  m.   f.  between  concentrated  zinc 
amalgams  does  not  agree  with  the  Nernst  formula  : 


1  Zeit.  f.  phys.  Chem.,  71,  625. 


26 

RT1  cl 

TT  =  -T=-  In  -1. 
nF       c2 

Cady1  claimed  that  this  discrepancy  was  due  to  the  heat  of 
dilution  of  the  amalgams  and  proposed  the  formula 

U    ,  RT,    cv 
*-&+&*% 

On  comparing  this  equation  with  the  well-known  Gibbs-Helm- 
holtz  equation, 

u  .  T* 

=  nF^    rdT 

we  see  that  if  the  equation  of  Cady  holds  true, 

dn        R        c. 


as  was  pointed  out  by  Cady  himself. 


In  the  following  table  the  values  of  -.^  obtained  experi- 
mentally are  compared  with  those  calculated  from  the  above 
relation. 

dx  dn 


Concentration. 


Percent 


Lewis 


2/IOO-I  .8/100 

o  .  0000046 

o  .  0000045 

o  .  0000045 

o  .  0000043 

2/IOO-I  -4/100 

0.0000162 

0.0000154 

0.0000151 

0.0000146 

2/100-   I/IOO 

0.0000295 

0.0000298 

0.0000294 

0.0000286 

2/IOO-O.5/IOO 

0.0000590 

0.0000597 

0.0000591 

0.0000579 

.8/100-1  .4/100 

0.0000116 

0.0000108 

0.0000106 

0.0000103 

.8/100-  i/ioo 

o  .  0000249 

0.0000253 

0.0000250 

o  .  0000243 

.8/100—0.5/100 

o  .  0000544 

0.0000552 

o  .  0000546 

0.0000535 

.4/100-  i/ioo 

0.0000133 

0.0000146 

0.0000143 

0.0000140 

.4/100-0.5/100 

0.0000428 

o  .  0000443 

o  .  0000440 

o  .  0000433 

1/100-0.5/100 

0.0000295 

0.0000298 

0.0000296 

0.0000292 

Expression  of  Concentration. — In  calculating  the  values 
for  the  last  three  columns,  the  following  values  were  given  to 
the  constants:  R  ==  8.316;  N  =  2;  F  :=  96,540.  There  has 
been  considerable  discussion  as  to  the  best  method  of  express- 
ing concentrations  in  a  case  of  this  kind.  In  the  last  three 
columns  three  different  methods  of  expressing  concentrations 
are  employed,  and  since  the  concentrations  employed  are 


1  Jour.  Phys.  Chem.,  2,  551  (1898). 


27 

fairly  great  the  different  methods  give  considerably  different 
results.  In  the  first  calculated  column  the  concentrations  are 
expressed  as  grams  of  zinc  in  grams  of  mercury.  In  the 
second,  the  concentrations  are  expressed  as  grams  of  zinc  in 
grams  of  amalgam.  In  the  last  column,  the  concentrations  are 
expressed  according  to  the  method  suggested  by  Lewis,1 
which  depends  on  the  generalization  that  the  activity  of  a 
substance  is  proportional  to  its  mol  fraction.  That  is,  in- 
stead of  calculating  ^  from  the  expression  -^  In  ~,  the  ex- 

R  n          in 

pression  — =  In  -  —TIT  /  -   -^-r-  is   used,   where  n  =  the   num- 

nF       n  +  Nj  /  n  +  N2 

ber  of  molecules  of  dissolved  substance,  and  Nx  and  N2 
the  number  of  molecules  of  the  solvent  in  the  two  amalgams 
respectively.  On  comparing  the  values  in  the  three  calcu- 
lated columns  with  those  in  the  observed  column,  it  is  seen 
that  the  values  in  the  second  calculated  column  are  in  much 
better  agreement  with  the  facts  than  either  of  the  other  two. 
Of  course,  as  the  dilution  increases  the  three  calculated  values 
approach  each  other,  and  in  the  case  of  fairly  dilute  amalgams, 
it  makes  no  difference  as  to  which  of  the  three  methods  of 
expressing  the  concentration  is  used.  It  was  probably  due 
to  the  fact  that  Richards2  and  his  co-workers  did  not  employ 
very  concentrated  amalgams  that  they  failed  to  detect  these 
really  considerable  differences  due  to  the  different  methods  of 
expressing  the  concentration.  With  the  exception  of  the 
pairs  of  amalgams  containing  the  one  of  1.4/100  concentra- 
tion (which  for  some  reason  seems  to  have  given  an  incorrect 
value),  all  the  values  in  column  2  agree  with  the  observed 
values,  within  the  experimental  error.  It  appears,  then,  that 

the  value  of  -^  for    zinc   amalgams  may  be  obtained  from 

dit        R  ,     c. 

the  relation  ^  =  -^  In  —  ,     if    the    concentrations     are    ex- 
dT      nF       c2 ' 

pressed  as  grams  of  zinc  in  grams  of  amalgam,  as  accurately 

1  Jour.  Amer.  Chem.  Soc.,  30,  668  (1908). 

2  Publication  No.  118,  Carnegie  Institution  of  Washington. 


28 


as  from  any  experimental  methods  now  within  our  reach. 

The  Gibbs-Helmholtz  Equation.  —  From  the  data  already 
given,  a  great  many  heats  of  dilution  of  zinc  amalgams  may 
be  calculated  by  means  of  the  Gibbs-Helmholtz  equation 


Concentration  TT  ^-  Q  calc.  at  25° 

2/100  -   I/IOO  0.006905  0.0000294  -  85.56 

2/IOO-0-5/IOO  0.014730  0.0000590  —132.81 

i.  8/100-  i/loo  0.005966  0.0000249  -67.03 

1.8/100-0.5/100  0.013791  0.0000544  —111.56 

i/ioo  -0.5/100  0.007825  0.0000296  -  45.92 

O.2/IOOO—   I/IOOO     O.OO87O2     O.OOOO298  8.21 

As  was  to  be  expected,  the  heat  of  dilution  grows  less  as 
the  dilution  increases,  and  beyond  the  dilution  of  i/iooo  it 
should  become  equal  to  zero  since  the  potentials  of  zinc  amal- 
gams obey  the  gas  laws  beyond  that  dilution.  The  actual 
measurement  of  the  heat  of  dilution  is  exceedingly  difficult, 
on  account  of  the  experimental  difficulties  involved.  There 
is  little  doubt  that  all  of  the  results  obtained  by  previous 
workers  have  been  seriously  in  error.  Unless  all  oxidation  of 
the  zinc  is  prevented,  the  results  cannot  be  relied  upon,  since 
the  heat  of  oxidation  of  zinc  is  so  great  that  a  proportionally 
small  amount  of  oxidation  would  neutralize  any  cooling  effect 
caused  by  the  dilution.  A  special  study  has  been  made  of 
these  conditions,  and  it  is  hoped  that  the  results  will  be  ready 
for  publication  in  the  near  future. 

The  Reduction  of  Zinc  by  Mercury 

If  a  mixture  of  two  metals  be  brought  into  a  solution  of 
their  salts,  one  metal  will  go  into  solution  while  an  equivalent 
amount  of  the  second  will  be  precipitated  until  the  equilibrium 
condition  is  reached.  The  condition  of  equilibrium  is: 

A  T-*          i       •**•  -*"      1  •*!         I         T^  R  JL  |  A    2 

A—  FI  +  -   -  In  -     +  F2  -          In  --  =  o 
n,        pl  n,        p, 

the   well-known   equation   derived   by   Nernst1   in    which    P, 


Zeit.  phys.  Chem.,  22,  539  (1897). 


29 

represents  the  solution  tension  of  the  first  metal,  pl  the  osmotic 
pressure  of  its  ions  in  solution,  nl  its  valence,  Fx  the  difference 
in  potential  between  the  metal  and  the  solution;  the  same 
letters  with  the  subscript  (2)  refer  to  the  second  metal.  A, 
represents  the  difference  of  potential  between  the  two  metals, 
R,  the  gas  constant,  and  T,  the  absolute  temperature.  In 
the  case  of  an  amalgam  Fx  =  F2  and  A  =  O,  so  that  the  above 
equation  reduces  to  : 


"l  Pl  "2  P2 

Ogg1  has  verified  the  above  equation  in  the  system  mer- 
cury, silver  nitrate  and  water.  In  this  case  the  equilibrium 
concentrations  are  of  sufficient  magnitude  to  be  determined 
analytically.  Hulett  and  De  Lury2  found  evidence  of  a  defi- 
nite equilibrium  in  the  system  mercury,  cadmium  sulphate 
and  water. 

If  mercury  is  shaken  with  a  zinc  sulphate  solution  we  may 
expect  some  mercury  to  go  in  solution  and  some  zinc  to  be 
precipitated  into  the  mercury  forming  a  very  dilute  amalgam. 
As  in  the  case  of  cadmium  the  concentrations  involved  in  this 
equilibrium  are  too  small  to  be  determined  by  chemical  means, 
but  since  the  e.  m.  f  .  between  the  two  zinc  amalgams  is  propor- 
tional to  their  relative  concentrations  it  was  thought  that  a 
determination  of  the  potentials  of  very  dilute  amalgams 
would  indicate  at  least  the  order  of  magnitude  of  these  equi- 
librium concentrations. 

Cell  No.  2  with  the  following  amalgams  at  25°  gave: 

2  phase  b  c 

2.220  2  I 


1  0.026768  0.035465 

2  0.026764  0.035470 
15                               0.026773  0.035479 

The  —    amalgam    remained    practically  constant    at 


1  Ibid.,  27,  285  (1898). 

2  Jour.  Am.  Chem.  Soc.,  30,  1805  (1908). 


30 

0.03547°  f°r  a  week,  which  was  the  same  value  as  was  observed 
for  an  amalgam  of  the  same  concentration  in  cell  No.  i. 

Cell  No.  6  had  six  compartments.  One  compartment  con- 
tained the  standard  electrode,  the  next  four  contained  weighed 
amounts  of  mercury,  and  into  the  sixth  were  put  60.7021 

grams  of  mercury  and  0.00607  gram  of  zinc  making  a  — < 
amalgam.  The  cell  was  then  sealed  off  and  put  at  25°.  The 
e.  m.  f .  of  this  — -4  amalgam  against  the  constant  electrode  was : 

5  hours  0.064937 

7  hours  0.064995 

10  hours  0.065000 

i  day  0.065226 

There  is  some  evidence  of  a  drift  here,  but  the  e.  m.  f.  was 
practically  constant  at  0.065000  for  some  time. 

The  cell  was  now  taken  out  of  the  thermostat  and  was 
rocked  and  the  wire  glowed  for  several  days  to  remove  any 
residual  oxygen  before  depositing  the  mercury  in  the  other 
compartments  to  form  the  more  dilute  amalgams.  The  cell 
was  then  warmed  up  to  about  35°  before  it  was  put  back 
in  the  25°  bath  as  it  had  been  found  that  the  two-phase 
electrode  came  to  equilibrium  more  quickly  from  the  higher 
temperature. 

In  depositing  zinc  in  the  mercury  a  storage  battery  of 
1 20  volts  was  closed  over  large  external  resistances  which 
included  an  accurately  calibrated  set;  after  the  circuit  was 
closed  the  fall  of  potential  over  1000  ohms  was  measured  with 
the  potentiometer.  The  internal  resistance  was  so  small  that 
the  current  was  practically  the  same  before  and  after  the  cell 
had  been  switched  into  the  circuit,  so  that  the  time  necessary 
for  each  deposition  was  calculated  before  the  cell  was  switched 
into  the  circuit.  The  time  was  taken  with  a  stop  watch  and 
the  fall  in  potential  over  1000  ohms  was  measured  at  frequent 
intervals  during  the  time  the  current  was  depositing  zinc. 

Compartment  (6)  contained  35.0004  grams  of  mercury 
and  it  was  desired  to  deposit  0.00035  gram  of  zinc  from  the 
two-phase  electrode. 


At    9.15  circuit  was  closed. 

"     9.16  fall  of  potential  over  1000  ohms  was  0.19947  v. 

From    9.20  to  9.50       "  "  "  "  0.19941  v. 

At    9.55  0.19945  v. 

"   10.05  "  0.19950  v. 

"   10.10  "                "  "  "  0.19958  v. 

"   10.15  "                "  "  "  0.19970  v. 

"   10.18  "  "  "  0.19993  AT. 

"    10.20  "  "  "  "  0.20010  V. 

"  10.25                     "  "                      "            "           0.20018  v. 

"  10.30                    "  "                      "            "           0.20004  v. 

"  10.35                     "  "                      "            "           0.20008  v. 

"  10.38  0.20015  v. 

"  10.41  broke  the  circuit. 

The  average  value  of  the  current  was  0.00019964  which 
flowed  for  one  hour,  twenty-six  minutes  or  5160  seconds  and 
gave  1.0301  coulomb.  As  one  coulomb  deposits  0.0003385 
gram  of  zinc  there  was  deposited  in  the  35  grams  of  mercury 
0.0003486  gram  of  zinc. 

Immediately  after  the  circuit  was  broken  the  cell  was 
rocked  and  the  measurements  of  this  amalgam  against  the 
constant  electrode  were  taken. 

i  minute  0.094707 

5  minutes  0.094705 

ii  minutes  0.094704 

10  hours  0.094883 

15  hours  0.094944 

There  was  no  drift  here  for  several  hours.  The  value 
0.09470  is  evidently  the  correct  one  for  this  i/io5  amalgam 
(the  current  was  broken  somewhat  too  soon  so  that  the  con- 
centration of  this  amalgam  was  really  o.996/io5). 

Zinc  was  now  deposited  in  the  34.9902  grams  of  mercury 
in  compartment  c.  The  current  was  0.00011814  which 
remained  absolutely  constant  for  the  fourteen  minutes  and 
thirty-five  seconds  it  was  depositing  zinc.  This  gave  0.10336 
coulomb  which  deposited  0.00003487  gram  of  zinc.  It  was 
found  necessary  to  rock  the  cell  for  about  five  minutes  before 
the  readings  became  at  all  constant. 


32 

5  minutes  o.  12448 

10  minutes  o.  12453 

15  minutes  o.  12462 

20  minutes  o.  12471 

25  minutes  o.  12471 

35  minutes  0.12480 

There  is  evidence  of  considerable  loss  here.  The  original 
value  of  about  0.1245  is  probably  near  the  correct  one. 

Compartment  d  contained  34.9889  grams  of  mercury.  A 
current  of  0.0001175  was  passed  for  88  seconds  which  gave 
0.010341  coulomb  and  deposited  0.000003500  gram  zinc. 
The  cell  was  rocked  for  two  minuts  and  then  measured. 

2  minutes  o.  151  + 

3  minutes  o.  153  + 

4  minutes  0.1550 

5  minutes  0.1555 

6  minutes  o.  1558 

7  minutes  o.  1562 
10  minutes  o.  1575 
25  minutes  o.  1587 
12  hours  o. 1664 

The  potential  of  this  i/io7  amalgam  was  variable  during 
the  first  few  minutes  but  at  the  end  of  four  minutes  it  became 
constant  enough  to  be  followed  in  the  fourth  place  decimal. 

The  value  0.1550  may  be  taken  as  an  approximation  to 
the  correct  value. 

Compartment  e  contained  34.9849  grams  of  mercury.  A 
current  of  0.00001386  amp.  for  75V5  seconds  gave  0.0010423 
coulomb  and  deposited  0.0000003525  gram  of  zinc.  The  cell 
was  then  rocked  for  one-half  minute  and  measured. 

1  minute  0.23+  7  minutes  0.435 

2  minutes  0.35+  25  minutes  0.479 

3  minutes  0.40+  30  minutes  0.511 

4  minutes  0.41  4  hours  0.7031 

The  behavior  of  this  i/io8  amalgam  was  decidedly  differ- 
ent from  any  previous  one.  The  amalgam  never  became 
constant  and  in  four  hours  the  value  had  risen  to  0.703  which 
is  about  the  value  that  pure  mercury  falls  to  when  allowed  to 
stand  in  an  oxygen  free  zinc  sulphate  solution. 


33 

A  great  many  measurements  have  been  made  on  pure 
mercury  against  the  two-phase  standard  in  these  cells,  the 
same  precautions  being  used  to  exclude  oxygen  as  in  the  case 
of  the  dilute  amalgams.  The  value  at  first  was  about  1.09 
but  in  time  settled  down  to  about  0.74,  usually  something  less 
than  0.75.  After  four  hours,  then,  there  was  no  more  zinc  in 
the  i/io8  amalgam  than  there  was  before  any  zinc  had  been 
deposited  by  the  current. 

After  three  days  zinc  was  again  deposited  in  this  mercury 
in  compartment  e.  It  was  thought  probable  that  the  amal- 
gam had  lost  some  zinc  while  the  current  was  still  depositing 
it,  so  a  larger  current  was  used.  0.000123  ampere  for  84/5 
seconds  gave  0.001084  coulomb  and  deposited  0.0000003669 
gram  of  zinc. 

After  i  minute  the  e.  m.  f.  was  o.  18  + 
After  2  minutes  the  e.  m.  f.  was  o.  19 
After  3  minutes  the  e.  m.  f.  was  0.21 
After  5  minutes  the  e.  m.  f.  was  0.25 
After  35  minutes  the  e.  m.  f.  was  0.50 
After  2  days  the  e.  m.  f.  was  0.714 

In  this  case  where  the  time  required  to  deposit  the  zinc 
was  only  about  i/io  of  that  previously  used,  a  much  lower 
value  was  obtained  for  the  initial  reading.  This  is  also  in 
good  agreement  with  the  idea  of  an  equilibrium  between  the 
mercury  and  zinc  sulphate,  since  the  zinc  from  the  first  i/io8 
amalgam  would  have  reduced  the  concentration  of  mercury 
ions  in  solution  and  so  there  would  have  been  fewer  present 
to  remove  the  zinc  from  the  second  i/io8  amalgam. 

After  three  days  this  amalgam  had  apparently  lost  all  of 
its  zinc.  Zinc  was  again  deposited  in  this  mercury  by 
0.00000235  amp.  for  434/5  seconds,  which  gave  0.00010315 
coulomb  and  deposited  0.00000003502  gram  of  zinc.  The  cell 
was  rocked  during  the  whole  time  of  deposition : 

1  minute  o .  24 

2  minutes  0.27 
5  minutes                                     0.408 

30  minutes  0.514 


34 

In  a  few  hours  the  value  rose  to  the  equilibrium  value  of 
about  0.7  +  . 

In  the  following  table  the  e.  m.  f.  measurements  are  given 
between  amalgams  varying  in  concentration  from  that  satu- 
rated at  25°  to  one  containing  one  part  of  zinc  in  one  billion 
parts  of  mercury.  In  the  column  marked  observed  the  value 
of  each  amalgam  measured  against  the  two-phase  is  given. 


Amalgams 
2  .  22O/IOO 
2/100 

i.  8/100 
I  .4/100 

I/IOO 

5/iooo 

2/1000 
I/IO3 
I/IO4 
I/IO5 
I/IO6 
I/IO7 
I/IO8 
I/IO9 

Observed  V 
o  .  oooooo 
o  .  000890  • 
0.001829 
0.004260 

0.007795 

0.015620 
0.026768  • 

0.035470 

0.065000 
o  .  094700 
o.  12450 

0.1550 
0.185 

0.24 

Differences 

o  .  000890 
0.00093$ 
0.002431 

0-003535 
0.007825 
0.011148 
0.008702 
0.029530 
0.029700 
0.029800 
o  030500 
o  .  0300 

0.0550 

Calculated 

0.001340 
0.001351 
0.003226 
0.004320 
0.008898 
0.029560 
0.029560 
0.029560 
0.029560 
0.029560 
0.029560 
0.029560 
0.029560 

The  Nernst  equation  for  concentration  cells  is  E  =  =  RT 
In  Cj/C2  where  Cl  and  C2  in  our  case  represent  the  concentra- 
tion of  the  zinc  in  the  mercury.  Changing  to  Briggs'  log- 
arithms for  25°  we  get  V  =  0.029560  log  Cj/C^ 

As  has  been  observed  by  previous  workers  the  e.  m.  f. 
between  the  concentrated  amalgams  is  much  too  small  to 
agree  with  the  above  equation.  There  is  very  good  agreement 
in  the  range  from  2/1000  to  i/io7  and  even  to  i/io8  if  the 
first  reading  is  taken. 

In  the  case  of  amalgams  more  dilute  than  i/io8  it  was 
impossible  to  take  any  measurements  before  the  amalgam  had 
lost  part  of  its  zinc.  In  fact,  the  potential  measurements 
seem  to  leave  no  doubt  that  in  a  short  time  both  the  i/io8 
and  the  i/iofl  amalgams  had  lost  all  of  the  zinc  that  had  been 
deposited  in  them.  It  is  well  known  that  oxygen  dissolved 
in  the  solution  about  a  zinc  amalgam  does  remove  the  zinc. 
There  is  little  doubt  that  the  glowing  platinum  wire  in  the  cell 


35 

removed  all  oxygen  from  the  vapor  phase  and  it  seems 
that  the  agitation  of  the  electrolyte  for  days  with  the 
two-phase  amalgam,  which  contained  over  200  million 
times  as  much  zinc  as  the  i/io8  amalgam,  would  have  re- 
moved any  residual  oxygen  from  the  solution.  Considering  all 
of  these  points  it  seems  very  unlikely  that  zinc  was  removed 
from  these  very  dilute  amalgams  by  oxygen. 

The  equilibrium  between  mercury  and  zinc  in  the  amal- 
gam and  in  solution  undoubtedly  follows  the  relation  : 


where  Px  and  P2  represent  the  concentrations  of  zinc  and  mer- 
cury in  the  amalgam  and  />x  and  p2  their  respective  concentra- 
tions in  solution.  In  our  case,  P2,  the  concentration  of  mer- 
cury in  the  amalgam,  and  plf  the  concentration  of  zinc  in  so- 
lution, are  constant.  The  mercury  in  solution  is  mercurous 
mercury,  but  as  has  been  shown  by  Ogg1  the  ions  are  com- 
posed of  two  atoms  each  and  carry  a  double  charge  so  that 
n2  ==  i  and  for  zinc  nt  --=  2.  The  equation  reduces  to 
Pt  X  p2  ==  K  and  K  is  evidently  a  very  small  number.  If 
mercury  is  brought  into  a  zinc  sulphate  solution  mercury 
should  go  into  solution  and  zinc  be  deposited  in  an  equiva- 
lent quantity  in  the  mercury  until  the  equilibrium  constant 
has  been  satisfied.  The  measurements  of  the  e.  m.  f.  between 
pure  mercury  and  the  two-phase  electrode  have  indicated 
this  very  thing.  The  initial  high  value  of  about  1.09  has 
always  settled  down  to  about  0.7  +  indicating  a  deposition 
of  zinc  in  the  mercury.  This  reaction  must  be  electrolytic  so 
that  when  one  double  gram  atom  of  mercury  has  passed  into 
solution  a  gram  atom  of  zinc  has  been  deposited.  It  is  thus 
seen  that  the  actual  concentration  of  zinc  in  the  mercury  is 
dependent  upon  the  relative  volumes  of  mercury  and  solution, 
at  the  same  time  the  reaction  must  proceed  until  P1  X  p2  ==  K. 
Before  any  zinc  was  deposited  in  the  mercury  in  cell  No.  6 

1  Loc.  cit. 


36 

the  e.  m.  f.'s  between  the  two-phase  amalgam  and  the  mer- 
cury in  the  different  compartments  were  measured.  It  may 
be  observed  that  this  same  gradation  of  values  has  been  found 
in  many  other  cases  so  that  this  may  not  be  considered  a 
chance  effect. 

A  B  CD  E  F 

Two  phase      Mercury         Mercury         Mercury        Mercury  i/io* 

0.97  I. 00  1.04  I. 01  X 

From  these  values  it  appears  that  the  mercury  nearest 
the  compartments  containing  amalgams  had  acquired  the 
most  zinc.  In  the  case  of  the  two-phase  amalgam,  Pl  of  the 
above  equation  is  constant  and  relatively  very  large  so  that 
/>2  must  be  relatively  very  small.  This  amalgam  would  then 
tend  to  remove  the  mercury  ions  thrown  into  solution  by  the 
pure  mercury  in  the  other  compartments  so  that  the  mercury 
in  the  compartments  near  by  would  have  to  continue  giving 
ions  to  the  solution  in  order  to  preserve  the  equilibrium. 

When  zinc  was  deposited  electrolytically  in  the  mercury, 
mercury  was  also  deposited  in  proportion  to  their  concentra- 
tions, but  since  the  solution  was  saturated  with  zinc  sulphate, 
while  the  concentration  of  mercury  was  exceedingly  small,  it 
is  safe  to  assume  that  the  zinc  deposited  corresponded  to  the 
coulombs  passed  even  in  the  most  dilute  amalgams.  The 
deposition  of  zinc  disturbed  the  equilibrium  and  the  zinc  began 
at  once  to  leave  the  amalgam  and  replace  the  mercury  ions  in 
solution  until  the  equilibrium  condition  was  again  reached. 
In  the  cell  used  where  the  volume  of  mercury  was  about  V6 
of  that  of  the  electrolyte,  the  equilibrium  e.  m.  f.  between 
mercury  and  the  constant  electrode  fell  to  about  0.7+  and 
could  be  reached  from  either  side. 

It  seems  that  the  concentrations  of  our  i/io8  and  i/io9 
amalgams  were  of  the  order  of  magnitude  of  the  concentration 
of  the  mercury  ions  in  solution.  As  was  to  be  expected,  this 
is  a  somewhat  greater  dilution  than  was  found  by  Hulett  and 
De  Lury1  in  the  case  of  cadimum  amalgams. 

1  Loc.  tit. 


37 

Summary 

The  chief  results  of  this  investigation  are: 

The  determination  of  the  solubility  of  zinc  in  mercury 
between  o°  and  30°,  represented  by  the  equation 
X  =  1.388  +  0.0242  t  +  0.000352  t2 

where  X  =  grams  of  zinc  in  100  grams  of  mercury,  and  t  = 
the  temperature. 

The  determination  of  the  relation, 

D  =  13.5340  —  0.0859  p  --  (0.00244  —  0.000067  p)  (t  —  25) 
(where  p  =  grams  of  zinc  in  100  grams  of  mercury,  D  =  den- 
sity of  the  amalgam,  and  t  =  the  temperature)  between  the 
density  and  composition  of  zinc  amalgams. 

Measurements  on  the  electromotive  forces  of  zinc  amal- 
gams at  25°  extending  from  the  saturated  amalgam  to  a  dilu- 
tion of  i  part  of  zinc  to  one  billion  parts  of  mercury.  The  ob- 
servations were  all  based  on  a  constant  reproducible  electrode 
and  so  are  directly  comparable. 

The  accurate  measurement  of  -=  for  zinc  amalgam  cells 

which  would  seem  to  indicate  that  ~^  can  be  very  accurately 

d  1 

calculated  by  the  equation 

diz        R       q 

5r  =  nFln^ 

if  we  express  the  concentrations  as  grams  of  zinc  in  grams  of 
amalgam.  It  is  shown  that  for  this  calculation  this  method 
of  expressing  concentrations  is  superior  to  any  other  proposed. 
The  discovery  of  a  region  between  one  gram  of  zinc  to 
1000  grams  of  mercury,  and  one  gram  of  zinc  to  10  million 
grams  of  mercury  where  the  potential  between  any  two  amal- 
gams conforms  to  the  requirements  or  the  gas  laws.  Beyond 
this  dilution  the  potential  fell  off  so  rapidly  with  the  time  that 

no  accurate  measurements  could  be  taken,  which  indicates 

++  ++ 

that  the  reaction  Zn  +  Hg2  ~^~>  Zn  +  2Hg  is  a  reversible  one 

and  that  a  definite  equilibrium  is  established  in  the  system 
mercury,  zinc  sulphate  and  water. 


ACKNOWLEDGMENT 

My  sincere  thanks  are  due  to  Professor  G.  A.  Hulett  for 
suggesting  this  research  and  for  very  valuable  advise  offered 
during  the  progress  of  the  work. 


AN  INITIAL  FINE  OF  25  CEN 

WILL   BE   ASSESSED    FOR    FAILURE  TO   RET 
THIS    BOOK    ON    THE    DATE    DUE.    THE   PEN> 
WILL   INCREASE  TO   5O  CENTS  ON   THE  FOL 
DAY     AND     TO     $1.OO     ON     THE     SEVENTH 
OVERDUE. 


•>'•'•  ' 
OCT   7  1936 

Uvl 

JAW  27  1947 

- 

ISApr'SIWK 

5             l2Anrrrfi  it 

nf"  3  1  L  U 

i  FE8  26  1977 

. 

LD  21-1 

